A conventionally proposed power supply device of this type is such that in increasing an electric power from a DC power supply with a converter constituted by two transistors and a reactor and supplying the electric power to an electrical apparatus, an on-duty of an upper arm of the two transistors of the converter is set to 1.0 when a voltage command provided to the electrical apparatus is close to a voltage of the DC power supply (for example, see Patent Document 1). In this device, the on-duty of the upper arm is set to 1.0 when the voltage command provided to the electrical apparatus is close to the voltage of the DC power supply, thereby preventing vibration of an output voltage caused by the fact that the on-duty of the upper arm cannot be ensured because of the need for ensuring a dead time for preventing the two transistors from being simultaneously turned on.    [Patent Document 1] Japanese Patent Laid-Open No. 2005-51895